Optical disc apparatuses for irradiating a light beam onto an optical disc such as a CD (compact disc), a DVD (digital versatile disc) or a Blu-ray disc (trademark, to be referred to as BD hereinafter) and reproducing information by reading reflected light thereof have been spreading.
Such optical disc apparatus are made to record information by irradiating a light beam onto an optical disc and changing the local reflectance of the optical disc.
As for optical discs, it is known that the size of the light spot formed on the optical disc is defined substantially as λ/NA (λ: the wavelength of the light beam, NA: the numerical aperture) and the resolution is proportional to this value. For example, Non-Patent Document 1 shows details of a BD that can record about 25 GB of data on an optical disc having a diameter of 120 mm.
Meanwhile, optical discs are designed to record various pieces of information such as various kinds of contents including music contents and video contents or various data for computers. Particularly, in recent years, the quantity of information has increased due to the use of high definition images and high sound quality music and, because an increased number of contents is required to be recorded on a single optical disc, there is a demand for larger capacity optical discs.
Thus, techniques for increasing the recording capacity of a single optical disc by laying a recording layer on another have been proposed (refer to, e.g., Non-Patent Document 2).
On the other hand, optical disc apparatus employing holograms have been proposed as techniques for recording information on optical discs (refer to, e.g., Non-Patent Document 3).
For instance, as shown in FIG. 1, an optical disc apparatus 1 is designed to once converge a light beam from an optical head 7 in an optical disc 8 that is made of a photopolymer whose refractive index changes according to the intensity of emitted light and then converges the light beam once again to the same focus position from the opposite direction by means of a reflection device 9 arranged at the rear surface side (the lower side in FIG. 1) of the optical disc 8.
The optical disc apparatus 1 causes a light beam that is a laser beam from a laser 2 to be emitted, modulates the light wave by means of an acousto-optic modulator 3 and converts the light beam into collimated light by a collimator lens 4. Subsequently, the laser beam is transmitted through a polarization beam splitter 5 and converted from linearly polarized light into a circularly polarized light by a quarter wave plate 6 before it is entered into the optical head 7.
The optical head 7 is adapted to record and reproduce information. It reflects the light beam by a mirror 7A, focuses the reflected light beam by objective lens 7B and then emits the light beam onto an optical disc 8 that is driven to rotate by a spindle motor (not shown).
At this time, the light beam is once focused in the inside of the optical disc 8 before it is reflected by the reflection device 9 arranged at the rear surface side of the optical disc 8 and then converged to the same focus in the inside of the optical disc 8 from the rear surface side of the optical disc 8. Note that the reflection device 9 is formed by a condenser lens 9A, a shutter 9B, a condenser lens 9C and reflection mirror 9D.
As a result, as shown in FIG. 2(A), a standing wave is generated at the focus position of the light beam and a recording mark RM that is a small hologram of the size of a light spot having a shape of two cones that are bonded to each other at the bottoms thereof as a whole is formed. Then, the recording mark RM is recorded as information.
When the optical disc apparatus 1 records a plurality of such recording marks RM in the inside of an optical disc 8, it forms a mark recording layer by rotating the optical disc 8 and arranging the recording marks RM along concentric or spiral tracks. Then, the optical disc apparatus 1 can record recording marks RM so as to lay a plurality of mark recording layers one on the other by adjusting the focus position of the light beam.
As a result, the optical disc 8 shows a multilayer structure having a plurality of mark recording layers inside. For example, the optical disc 8 shows a distance (mark pitch) p1 between recording marks RM of 1.5 μm, a distance (track pitch) p2 between tracks of 2 μm and a distance p3 between layers of 22.5 μm as shown in FIG. 2(B).
Additionally, when the optical disc apparatus 1 reproduces information from an optical disc 8 where recording marks RM are recorded, it closes the shutter 9B of the reflection device 9 so that no light beam may be emitted to the optical disc 8 from the rear surface side thereof.
At this time, the optical disc apparatus 1 emits a light beam onto a recording mark RM in the optical disc 8 by the optical head 7 and causes the reproduction light beam generated from the recording mark RM to enter the optical head 7. The reproduction light beam is converted from circularly polarized light into a linearly polarized light by the quarter wave plate 6 and reflected by the polarization beams splitter 5. Additionally, the reproduction light beam is converged by a condenser lens 10 and emitted onto a photodetector 12 by way of a pin hole 11.
At this time, the optical disc apparatus 1 detects the quantity of light of the reproduction light beam by the photodetector 12 and reproduces information according to the results of the detection.    Non-Patent Document 1: Y. Kasami, Y. Kuroda, K. Seo, O. Kawakubo, S. Takagawa, M. Ono and M. Yamada, Jpn. J. Appl. Phys., 39, 756 (2000).    Non-Patent Document 2: I. Ichimura et al., Technical Digest of ISOM'04, pp 52, Oct. 11-15, 2005, Jeju Korea.    Non-Patent Document 3: R. R. McLeod et al., “Microholographic multilayer optical disc data storage,” Appl. Oct., Vol. 44, 2005, pp 3197.
Meanwhile, known optical disc apparatuses that correspond to CDs, DVDs or BDs have a possibility of producing surface shaking or eccentricity to the optical disc being driven to rotate but are made to be able to accurately emit a light beam onto the target track by performing various control operations such as focus control and tracking control according to the results of the detection of the light beam.
However, the optical disc apparatus 1 as shown in FIG. 1 is not adapted to be able to specifically perform operations such as focus control and tracking control so that it cannot accommodate surface shaking or eccentricity that can be produced on an optical disc.
In other words, the optical disc apparatus 1 may not be able to accurately focus a light beam to a desired position in an optical disc 8 and correctly record or reproduce information.